1. Field of the Invention
This invention relates to apparatus for sludge concentration for obtaining thickening of sludge by gravity settling, and, more particularly to an apparatus for sludge thickening capable of preventing floating of the sludge when thickening thereof is carried out.
2. Description of the Prior Art
Sludge disposed of in a municipal sewage plant is first thickened in a thickener, then subjected to digestion treatment, and thereafter dewatered. Processes for obtaining thickening of sludge known in the art include a gravity thickening process, floatation thickening process, and centrifugation thickening process. Of these processes, the gravity thickening process relying on the weight of the suspended solids or the difference in density between the suspended solids and the liquid in which they are suspended is the most simple process. Thus, this process is used in almost all the municipal sewage plants. The gravity thickening process essentially has the problem that although sludge settles, it floats in actual practice. Floatation of the thickened sludge is caused by increased buoyancy of the suspended solid particles due to adhesion of minuscule bubbles thereto as a result of generation of anaerobic gases. Floatation of the thickened sludge adversely affects the result achieved by a waste water treatment process for treating supernatant and reduces the efficiency of a sludge treatment process for treating the sludge.
In order to solve the aforesaid problem, the following report has been made: "REPORTS ON EXPERIMENTS CONDUCTED ON GRAVITY THICKENING" by K. Terada et al., Proceeding of the 15th Annual Conference of Japan Sewage Works Association (Japanese), Tokyo, May, 1978. Published by: Shadan-Hojin (Corporate Juridical Person) Nihon-Gesuido-Kyokai (Japan Sewage Works Ass'n)
The description in the above noted report can be summarized as follows. In carrying out gravity thickening of sludge, it is possible to delay the time at which floatation is initiated by subjecting the sludge to aeration. More specifically, by subjecting the sludge to aeration for 30-60 minutes, the time for floatation to begin can be delayed by over 20 hours. The time during which sludge stays in a thickener is 8-20 hours, so that floating of the sludge can be effectively prevented.
In the above noted report there is no specific mention of any concrete device for carrying out aeration, although it is stated therein that aeration has the effect of avoiding floatation. Thus, it is difficult to carry out aeration for practical purposes. Even if aeration of the sludge is carried out in the thickener in accordance with the description in the report in the literature, it would be difficult to obtain uniform aeration or mixing of the sludge in the thickener due to a large volume thereof and it would be impossible to prevent floating of the sludge with a high degree of efficiency. Moreover, a thickener is essentially intended to obtain settling of the sludge that is placed therein in a stationary fashion. Therefore, if the sludge is subjected to aeration, the operation would naturally be performed in a batch treatment and the speed at which the sludge is treated would be reduced.
Thus, one would be faced with the problem that difficulties would be encountered in carrying out thickening of the sludge effectively on the basis of the description in the aforesaid report.